A single panel scrolling color projection display system is characterized by a single light modulator panel such as a liquid crystal display (LCD) panel having a raster of individual picture elements or pixels, which panel is illuminated by horizontally elongated red, green and blue illumination bars or stripes. The stripes are continuously scrolled vertically across the panel while the illuminated rows of pixels are synchronously addressed with display information corresponding to the color of the then incident stripe. See, for example, U.S. Pat. No. 5,410,370, “Single panel color projection video display improved scanning” issued to P. Janssen on Mar. 25, 1994, and U.S. Pat. No. 5,416,514, “Single panel color projection video display having control circuitry for synchronizing the color illumination system with reading/writing of the light valve” issued to P. Janssen et al. on May 16, 1995.
Such single panel systems are to be distinguished from the more conventional three-panel systems, in which separate red, green and blue beams each fully illuminate and are modulated by a separate light modulator panel. The modulated beams are then superimposed on a display screen to produce a full color display. See, for example, U.S. Pat. No. 5,917,561, “Liquid-crystal image projecting apparatus having a color purity correction filter” issued to Hatanaka on Jun. 29, 1999.
Light engines for both single-panel and three-panel color projection display systems commonly utilize high intensity arc lamps to provide the level of intensity needed for a bright display, as well as dichroic filters to split the lamp light into red, green and blue components for modulation, and then to recombine the modulated components for projection display.
In both the single panel and the three panel systems of the prior art, the desire for high light efficiency has dictated that the optical path lengths of the red, green and blue beams are approximately equal. Otherwise, those beams which must travel farther from the light source to the display panel have a greater etendue (angular extent), and some of light from those beams is lost. See, for example, U.S. Pat. No. B1 4,864,390, “Display System with Equal Path Lengths”, issued to McKechnie et al. on Sep. 5, 1989.
Unfortunately, such systems, while efficient in terms of light utilization, require multiple relays of relatively high optical quality to create equivalent images for the three colors. In addition, thorough integration (mixing) of light in the preceding light collection stages is necessary. The large number of optical components contributes significantly to the size and overall cost of the system.
The illumination architecture for a presently used light engine 1 for a scrolling color projector is shown schematically in FIG. 1. White light from source S is split into a blue component B and a green/red component G/R by dichroic element 2. The B component is directed by lens 3 and mirror 4 to prism scanner 5. The G/R component is passed by dichroic element 2 through lens 6 to dichroic element 7, which splits the G/R component into a green component G and a red component R. The G component is reflected by element 7 to prism scanner 8, while the red component is passed through dichroic element 7 to prism scanner 9. The scanned R, G, B components are then directed to recombination dichroic elements 10 and 11 by mirror 12 and relay lenses 13 through 17.
Relay lenses 13 through 17 are designed to limit the light expansion over the long recombination path from the prism scanners 5, 8 and 9 to the output lens 18. Consequently, light that is telecentric at the prism scanners 5, 8 and 9 is not telecentric at the recombination dichroic elements 10 and 11. As a result, color shading is introduced over the scan (from the top to the bottom of the display) unless (expensive) shaded dichroics are used.
In accordance with the invention, at least the light engine portion of a projection display system employs loss-less etendue-preserving light guides, enabling a compact arrangement through the use of unequal path lengths for the separate light beams, while eliminating the need for many high quality optical lenses, and preserving the light efficiency of the equal path length designs of the prior art.
In accordance with one aspect of the invention, a light engine for a projection display system comprises: a beam splitter for splitting light from a source into two or more light components; and a light guide comprising:
at least a source branch for guiding light from the source to the beam splitter; and at least two component branches for guiding the light components away from the beam splitter.
In accordance with a preferred embodiment of the light engine, the beam splitter comprises crossed dichroic elements or splitting the source light into red, green and blue components.
In accordance with another preferred embodiment of the light engine, a scanning stripe generator is provided for the red, green and blue light components, and each component branch of the light guide guides one of the light components to one of the scanning stripe generators.
In accordance with another preferred embodiment of the light engine, a beam recombiner of crossed dichroic elements is provided for recombining the red, green and blue components.
In accordance with another aspect of the invention, a projection display system is provided, the system comprising a light engine of the invention, at least one light modulator panel for modulating light in accordance with a display signal; and a projection lens for projecting the modulated light onto a display screen.
In accordance with a preferred embodiment of the projection display system, a polarizing beam splitter (PBS) is provided between the light engine and the light modulating panel for transmitting light of a first polarization state and reflecting light of a second polarization state transverse to the first polarization state.